This invention is concerned with a hydrogen sensor including a solid hydrogen conducting electrolyte. Hydrogen sensors of this type are used to detect, and to measure, hydrogen when it is present in a gaseous system.
Several types of hydrogen monitors and detectors have been described, which utilise a variety of principles related to the physicochemical properties of hydrogen. Some of these devices are only useful at low hydrogen concentrations, a typical upper limit being of the order of 3-4%. Others are able to operate over a wider, and more useful, range of concentrations, but generally either have low sensitivity and thus cannot detect small changes, or exhibit a slow response time and thus cannot respond to transient changes. Further, some of these devices require a supply of pure hydrogen as a standard reference, and some of them only operate at elevated temperatures.
It has also been proposed to use solid sensors in gas detection devices.
Ichinose et al., in CA 1,078,019, describe a gas detecting element including a gas detecting body made of a zinc oxide based semiconductor with a catalytic material coated onto its surface. This device measures the variation of the surface resistance of the semiconductor material when in contact with the gas. This device appears to be limited to detecting a combustible gas such as iso-butane, but also refers to hydrogen and carbon monoxide.
In U.S. Pat. No. 4,636,294 there is described a device for the detection and measurement of hydrogen sulphide. This device requires an anode, a cathode, a solid electrolyte, and a reference electrode in contact with the electrolyte. The reference electrode should be shielded from the gas being tested. It is also suggested that xe2x80x9csacrificial reference electrodes such as silverxe2x80x9d can be used in such a device.
Alberti et al. in U.S. Pat. No. 5,453,172 describe a solid state gas sensor which comprises a solid protonic conductor sandwiched between a catalysing electrode on one side, and a solid state reference electrode on the other side. An end of the sandwich comprising these adjacent materials is exposed to the gas to be measured, and the emf generated across the outside layers of the sandwich is measured. This device is said to be suitable for hydrogen. The protonic conductor is preferably zirconium hydrogen phosphate, the catalysing electrode is preferably platinum or palladium, and the reference electrode is preferably titanium hydride or zirconium hydride. It is also indicated that there are drawbacks with using a silver electrode in contact with zirconium hydrogen phosphate which can lead to a total reduction of Ag+ in such a system.
Currie et al. in WO 94/28403 describe an integrated monolithic gas sensor, which comprises a substrate carrying several deposited thin films. The thin films include an electrically conductive heating element, a conductive reference electrode, and a second conductive electrode. These three are electrically isolated from each other. A thin film ionic conductor, and a thin film reactive gas sensitive layer are placed between the reference electrode and the second conductive electrode to form an electrolytic cell in which an electrolyte reaction including as reagent the gas to be detected produces an emf between the two electrodes indicative of the concentration of the gas. The sensor also includes a micro thermometer formed of a deposited thin film wire having a temperature dependant resistance. When the gas to be detected is carbon dioxide, the reactive gas layer may be sodium carbonate, and the ionic conductor may comprise a complex zirconium phosphate of the Nasicon type, of general formula Na3Zr2Si2PO12.
A process for the preparation of polycrystalline ceramic materials which conduct electricity by the mobility of hydronium ions or hydrogen ions is described by Kuriakose et al in U.S. Pat. No. 4,724,191. This process includes methods for preparing both the so-called Nazirpos family of compounds, which are complex sodium zirconium silicophosphates corresponding to the general formula Na(1+x)Zr2SixP(3xe2x88x92x)O12, and other complex materials including polyantimonic acids, alumina containing ceramics, and complex sodium silicates of the general formula Na5ReSi4O12, in which Re represents yttrium or gadolinium. Whilst it is stated that the polycrystalline materials obtained by the described process xe2x80x9care capable of use as a membrane in devices such as hydrogen fuel cells, hydrogen detectors, and steam electrolysersxe2x80x9d no preference is expressed amongst the many ceramics that can be made by the described process, and further there is no disclosure of how a device capable of both detecting gaseous hydrogen, and measuring the amount of hydrogen present, can be constructed.
A hydrogen concentration cell in which an electrolyte from the Nazirpos family is used has been described (J. Can. Ceramic Soc., 55, 34-37 (1986), and Solid State Ionics, 45,299-310 (1991)). This cell was constructed by applying platinum paint to both sides of a disc of the ceramic, curing it in hydrogen at about 100xc2x0 C., and sealing the disc onto the end of a glass tube with a silicone rubber resin. Electrical contact with the two platinised surfaces is stated to be obtained by means of a spring loaded platinum wire. This device functions as a concentration cell, in which one side (the side exposed within the glass tube) is exposed to hydrogen gas at a known partial pressure as a standard, and the other side of the cell is exposed to hydrogen gas either at a lower partial pressure, or in combination with another gas, for example nitrogen. Under these circumstances an emf is developed between the two electrodes indicative of the difference of the partial pressures of hydrogen at the two sides of the cell. This device has several disadvantages. It is difficult to ensure reliable electrical contact to the platinised ceramic surfaces with the spring loaded platinum contacts. The two sides of the ceramic disc have to be physically isolated from each other as different gas systems are in contact with each side of the cell. A reference gas of pure hydrogen at a known partial pressure also has to be provided.
This invention seeks to provide a reliable hydrogen detection device which is simple, easy to use, and which does not require any reference gas supply. Further, the device can be of reasonably rugged construction, and thus is potentially suitable for commercial exploitation. This invention utilises a disc comprising a solid state ceramic hydronium conductor of the Nazirpos family together with a silver based electrode system on one side, and a platinum electrode on the other. The device can thus be represented as the following electrochemical cell:
Pt,H2|H-Nazirpsio|Ag-Nazirpsio|Ag xe2x80x83xe2x80x831 
In this cell the overall reaction is:
xc2xdH2+Ag-Nazirpsioxe2x86x92Ag(s)+H-Nazirpsio xe2x80x83xe2x80x832 
Thus in a first broad embodiment this invention seeks to provide a hydrogen detection device comprising in combination:
(a) a body of phosphate bonded ceramic electrolyte of the general formula Na(H3O)xZr2SixP(3xe2x88x92x)O12 having a first face spaced apart from a second face;
(b) a catalytic noble metal electrode layer on the first face of the body in electrical contact with the ceramic electrolyte;
(c) a silver ion modified layer at the second face of the body;
(d) a silver electrode in contact with the silver ion modified layer; and
(e) conductive leads electrically connected to each of the faces; whereby the emf generated when the ceramic body is exposed to hydrogen gas can be measured.
Preferably, the first and second spaced apart faces on the ceramic body are substantially parallel to each other.
Preferably, the first and second faces are each substantially flat.
Preferably, the ceramic body is disc shaped, and the first and second faces comprise the two faces of the disc.
Preferably, the catalytic noble metal electrode layer is chosen from a layer of platinum or palladium. More preferably, the catalytic noble metal electrode layer is a layer of platinum.
Preferably, the silver electrode comprises a silver containing conductive cement applied over the silver ion modified layer.
Preferably, the two conductive leads are attached to each of the platinum layer and the silver electrode by means of a conductive cement. More preferably, the same silver conducting cement is used as both the silver electrode and to attach the two conductive leads.
Preferably, in the body of phosphate bonded ceramic electrolyte of the general formula Na(H3O)xZr2SixP(3xe2x88x92x)O12, x has a value of from about 1.3 to about 2.2.
Most preferably, in the body of ceramic electrolyte x has a value of about 1.5.